The present invention relates generally to seals for providing fluid sealing between a housing and a rotating shaft. More particularly, the invention relates to mechanical face seals in which a fluid is introduced between portions of the seal faces of the seal.
Conventional mechanical seals are employed in a wide variety of mechanical apparatuses to provide a pressure-tight and a fluid-tight seal between a rotating shaft and a stationary housing. The seal is usually positioned about the rotating shaft, which is mounted in and protrudes from the stationary housing. The seal is typically bolted to the housing at the shaft exit, thus preventing loss of pressurized process fluid from the housing. Conventional mechanical seals include face type mechanical seals, which include a pair of annular sealing rings that are concentrically disposed about the shaft, and axially spaced from each other. The sealing rings each have seal faces that are biased into physical contact with each other. Usually, one seal ring remains stationary, while the other ring contacts the shaft and rotates therewith. The mechanical seal prevents leakage of the pressurized process fluid to the external environment by biasing the seal ring sealing faces into physical contact with each other. As a result of the repeated physical contact between the faces, abrasion of the seal faces occurs and the seals typically exhibit undesirable wear characteristics and leakage.
The prior art attempted to overcome the above difficulties by employing non-contact mechanical seals that utilize a fluid interposed between the seal ring faces to reduce frictional wear. Conventional mechanical non-contact face seals typically employ pumping grooves, such as spiral or Raleigh step grooves, formed in one of the seal faces of the seal rings to develop a hydrodynamic lifting force that separates the seal faces. The resultant gap allows fluid to be disposed between the seal faces to prevent rubbing and abrasion of the seal faces.
Conventional non-contacting face seals, however, exhibit drawbacks in some areas of performance that are less than optimal. For example, in non-contacting seal designs, which rely primarily upon rotation to provide hydrodynamic separation of the seal faces, a substantial amount of seal face abrasion can occur during start-up operation or during periods when the shaft is rotating at low speeds. For this reason, these conventional non-contacting type mechanical face seals are unsuitable for low speed operation or for conditions which require frequent starting and stopping of the shaft.
In order to overcome the problems associated with purely hydrodynamic non-contacting seals, combination hydrostatic and hydrodynamic seals have been designed. Such combination seals typically rely on fluid pressure to provide hydrostatic separation and rotation to provide hydrodynamic separation of the seal faces. The performance of such combination seals has been less than optimal because the thickness of the fluid gap formed between the seal faces varies significantly depending upon rotation speed. Such conventional combination mechanical seals exhibit a substantial difference in fluid film thickness between hydrostatic (i.e., non-rotating) and hydrodynamic (rotating) operation due to the significant pumping force provided by the spiral grooves. The larger gap formed between the seal faces at high rotation speeds allows for greater leakage across the seal faces than would otherwise be desirable.
A significant number of conventional non-contacting seals employ a dual seal arrangement in which three or more seals rings are arranged axially along the shaft. Such dual seals can be arranged in a back-to-back, face-to-face, or tandem configuration and typically employ a high pressure barrier fluid at the outer diameter of the seal rings while maintaining the process fluid at the inner diameter of the seal rings. The barrier fluid is introduced to the seal faces through pumping grooves formed in one of the seal faces.
Dual non-contacting seals have also proven to be less than optimal in an number of areas of performance. Sealing the process fluid at the inner diameter of the seal can result in dirt or other particles within the process fluid becoming clogged between the seal faces, interfering with the operation of the seal. In addition, upon loss of the barrier fluid pressure, some dual seal designs do not maintain a fluid-tight seal, resulting in leakage of the process fluid. The additional seals rings in dual seals also result in an exceedingly bulky seal that is often unsuitable for applications in which the axially space along the shaft is limited. Furthermore, dual seals typically require modification of the housing to accommodate the increased size of the seal, resulting in complex and costly installation and servicing of the seal.
As the above described and other prior art seals have proven less than optimal, an object of the present invention is to provide an improved non-contacting mechanical face seal that is operable under a wider range of operating conditions.
Another object of the present invention is to provide a non-contacting mechanical face seal that maintains a fluid-tight seal that is less dependent on shaft speed.
Still another object of the present invention is to provide a non-contacting mechanical seal that minimizes seal face contact at lower shaft speeds and is suitable for applications requiring frequent starting and stopping of the shaft.
Yet another object of the present invention is to provide a non-contacting mechanical face seal that is compact in design and can be installed without modification of the housing.
A further object of the present invention is to provide a non-contacting mechanical seal that can provide the benefits of hydrostatic and hydrodynamic operation simultaneously.
Another object of the present invention is to provide a non-contacting mechanical seal having a sealing structure that alleviates O-ring hysteresis.
Other general and more specific objects of this invention will in part be obvious and will in part be evident from the drawings and the description which follow.